Bacteria lacking trans-translation activity have defects in development, differentiation, virulence, stress responses, and viability, but the reasons trans-translation is required for these processes are not known. Our long-term goal is to understand the mechanism of action and physiological role of trans-translation in bacteria. The overall objective of this application is to understand how regulation of trans-translation activity and substrate selectivity is used to control genetic pathways responsible for differentiation and cell cycle progression in Caulobacter crescentus. Our central hypothesis is that bacteria regulate the generation of key substrates for trans-translation, as well as the availability of tmRNA and SmpB, to control genetic circuits responsible for initiation of DNA replication and other physiological processes. The rationale for the proposed research is to establish a paradigm for post-transcriptional regulation through controlled trans-translation. The central hypothesis will be tested by pursuing the following specific aims: 1) identify the mechanism for generation of trans-translation substrates in C. crescentus; 2) identify mechanisms for regulation of trans- translation activity; and 3) determine the role of trans-translation in co-translational secretion. Under the first aim, genetic and biochemical approaches will be used to identify cis- and trans-acting factors responsible for substrate selectivity through a nucleic acid motif found in 66% of trans-translation substrates. In the second aim, genetic and biochemical assays will be used to identify the molecular interactions responsible for cell- cycle regulated SmpB proteolysis and tmRNA degradation by RNase R. In the third aim, the molecular basis for the genetic interaction between trans-translation and the SecYEG translocator will be tested. The proposed research is significant because it will provide a paradigm for post-transcriptional gene regulation by trans- translation that is required for bacterial growth and development. This paradigm will open the door to a more detailed understanding of how bacteria rapidly change their gene expression profiles in response to environmental and developmental cues. Elucidation of this process in a model system for bacterial development is expected to provide a framework for interpreting a wide array of data from other species, and a basis for understanding how trans-translation is used by bacteria in the environment and during pathogenesis.